1. Field of the Invention
This invention relates-to a resin composition having high-speed polymerizability and a plastic lens obtained therefrom. The lens obtained from the composition of the present invention has a refractive index (n.sub.d) of at least 1.59.
2. Description of the Related Art
Plastic lenses are lightweight and non-fragile compared with inorganic glass lenses so that they are finding rapidly increasing utility as eyeglass lenses, camera lenses or optical elements in recent years.
Resins widely employed these days for such purposes include a radical polymerization product of diethylene glycol bis(allyicarbonate) (for example, "CR-39", trade name; product of PPG). This resin has various advantages such as excellent impact resistance, lightweight, superb dyeability, and good workability such as good cutting and grinding machinability. Its refractive index, however, is 1.50 which is lower than the refractive index (1.52) of inorganic glass. To obtain optical characteristics comparable with those of a glass lens, it is necessary to increase the refractive index of the plastic lens, which unavoidably increases its central thickness or peripheral thickness, resulting in an overall large thickness. There is accordingly an outstanding desire for the development of a lens resin having a still higher refractive index.
The present inventors have already proposed a plastic lens made of a sulfur-containing urethane resin which can be prepared by the polymerization of a polyisocyanate and a polythiol (Japanese Patent Publication No. 15249/1992). The resin has a high refractive index (n.sub.d) of 1.57 or higher and also has various advantages like CR-39 so that it is used widely now. The resin, however, is accompanied with the problem that it requires long polymerization time and any attempt to complete the polymerization in a short time tends to result in a lens with optical unevenness (striae).
With a view toward improving the thermal deformation temperature, the present inventors have also proposed a composition which contains, in addition to the sulfur-containing urethane resin, a hydroxyl- or mercapto-containing acrylate, methacrylate or styrene (Japanese Patent Laid-Open No. 96208/1989). When a lens resin is prepared using the composition, it has an improved thermal deformation temperature. When polymerized at a high speed, however, the resin composition develops optical unevenness (striae) and in addition, cannot always provide a sufficient refractive index.
A composition capable of providing a plastic lens with an improved thermal deformation temperature and improved surface hardness in a similar manner has also been proposed (Japanese Patent Laid-Open No. 25240/1993). This composition has a relatively high refractive index but is accompanied with the drawback that due to the use of tolylene diisocyanate, it will be tinged yellow by ultraviolet rays in sun light as time goes on.